Modern electrical signal processing systems such as television receivers may utilize a video amplifier having an output capable of driving a selectively connected external load as well as a permanently connected internal load. This may produce problems due to the fact that the signal level at the output of the amplifier is sensitive to the presence or absence of the external load, and may vary.
One solution to this problem is to connect the external load to the amplifier via a switched jack. When an external load is not connected to the switched jack, a "dummy load" terminating impedance is applied across the output terminals of the jack. When an external load is connected to the switched jack, the dummy terminating impedance is disconnected from across the output terminals of the jack. Thus, the load presented to the amplifier is a constant impedance whether or not the external load is connected. This solution has an important drawback. Specifically, this solution requires the use of a switched output jack, and thus prevents the use of a standardized multiple pin connector such as an EIA connector, which is capable of coupling not only video signals but also additional desired signals.
It is desirable that the amplifier have a very low output impedance, preferably about zero ohms, in order to prevent interaction between the various loads which may be connected to the output of the amplifier. It is well-known that such a low output impedance can be obtained via the use of negative feedback, which generally reduces gain while improving frequency response and reducing output impedance.
An example of a prior solution to the problem of coupling the output of an amplifier to multiple loads is disclosed in U.S. Pat. No. 2,886,659 (Schroeder). Schroeder discloses the use of both negative voltage feedback and positive current controlled feedback in a relatively complex control system including a sensing resistor and a differential amplifier.